Does this mean that if you now decide to link a platform account (a new one without any progress) to an EA account, that this account will receive the progress of the merged accounts... or will this platform account have it's own separate progress?
Once you link the new platform to the EA ID that is cross-progressed, this platform will then have access to the progress and content of the cross-progressed account. Worth noting and stressing that any content on this new platform getting linked in will be lost.
If the account is “a new one” then there's likely nothing to be lost, but if you link in a platform that does have content and/or progress associated with it (because it’s already linked to another EA ID), this WILL be lost in favor of the cross-progressed account’s stats and content. This goes back to the “Unlinking can disassociate progress and content” aspect called out previously.